Zero point adjusting method for pressure detector of an injection molding machine and an apparatus therefor

ABSTRACT

A zero point adjusting method and apparatus for a pressure detector of an injection molding machine, capable of carrying out a zero point adjustment of the pressure detector when external loads acting on the pressure detector are completely removed without disassembling an injection mechanism. An advance limit position Qf and a retreat limit position Qb of a screw are detected by driving the screw with a low output to determine an intermediate position Pm. By moving the screw to the intermediate position Pm, a resin reaction force acting directly on the screw is removed. Reciprocating motion in which an amplitude decreases gradually is imparted to the screw, thereby causing a backlash, having equal plays in the advance and retreat directions, between a ball nut and a ball screw. Then, the zero point adjustment of the pressure detector is made.

FIELD OF THE INVENTION

The present invention relates to a method of adjusting a zero point of apressure detector of an injection molding machine and an apparatus forcarrying out the method.

DESCRIPTION OF THE RELATED ART

An injection molding machine, in which a pressure applied to a movablemember is detected by a pressure detector provided corresponding to themovable member, is known. For example, a machine, in which an injectionholding pressure or a back pressure in metering is sampled by a pressuredetector installed on the base of screw, is well known. The mostcommonly used pressure detector is a load cell, that is a pressuredetector comprising an elastic member and a strain gauge positioned on apower transmitting path between a movable member and a drive source ofthe movable member.

Since the strain gauge is designed to measure a deformation of metallicwire caused by pressure by changing the deformation into an electricresistance, a pressure applied to the movable member can be detected bydirectly affixing the strain gauge to the functionally necessary elasticmember positioned on a power transmitting path between the movablemember and the drive source or the movable member itself and the like.However, when the rigidity of such an elastic member is extremely highand therefore the deformation due to pressure is not sufficiently large,it is sometimes difficult to detect the pressure with high accuracy.Because of this problem, a member which is more elastically deformable,called a strain-sensitive member, is usually disposed on the powertransmitting path, and a strain gauge is affixed to this member in orderto constitute a pressure detector.

In general, the strain-sensitive member or the load cell body means theaforesaid member which is easily elastically deformable. Needless tosay, the strain gauge itself is also a kind of strain-sensitive member.

As an example of the pressure detector using the load cell, a generalconstruction of the pressure detector which is adapted to detectpressure applied to a screw shaft will be described briefly withreference to FIG. 1, which shows a cross section of a principal portionof an injection mechanism.

In FIG. 1, reference numeral 1 denotes a front plate of an injectionunit. Fixed to the front plate 1 are two or four tie rods 2 providedbetween the front plate 1 and a rear plate (not shown) positioned on theright side in the figure. A screw pusher plate 3 is slidably installedto the tie rod 2 through bushings etc.

A screw sleeve 4, which is rotatably installed to the screw pusher plate3 through two angular bearings, is rotated by a metering motor (notshown), which is fixed to the screw pusher plate 3 side, through atoothed pulley 5 fixed to the sleeve 4 and a timing belt (not shown)etc. set around the pulley 5.

An injection cylinder 6 fixed to the front surface of the front plate 1incorporates a screw 7 for metering and injection, and the base of thescrew 7 is fixed to the screw sleeve 4. The screw is rotated formetering and kneading in response to the rotation of the screw sleeve 4.

Further, a ball screw 8, which is used as a screw for motion, isattached to the rear plate, not shown, stationarily in the axialdirection but rotatably around the axis. The ball screw 8 can be rotatedby an injection motor fixed on the rear plate side through a timing beltand the like. The front end portion of the ball screw 8 protruding fromthe rear plate toward the front plate 1 is screwed into a ball nut 9(also called a socket) stationarily installed on the back surface of thescrew pusher plate 3. As the ball screw 8 rotates, the screw pusherplate 3 and all the members attached to the plate 3 as a whole move backand forth with respect to the front plate 1, and the screw 7 fixed tothe screw sleeve 4 moves back and forth in the injection cylinder 6.

A load cell 10, which constitutes a pressure detector for detecting aninjection holding pressure or a back pressure acting on the screw 7, isdisposed at the base of the screw 7, more particularly, between thescrew pusher plate 3 and the ball nut 9. The load cell 10 is composed ofa load cell body (also called a strain-sensitive member) fixed to thescrew pusher plate 3 and ball nut 9 by bolts 13 and 14, respectively,and a strain gauge 12 affixed to the load cell body 11. The load cellbody 11 is of a ring shape having a peripheral groove forming athin-wall portion 11a along the inner circumferential direction as shownin the side sectional view of FIG. 1. The load cell 10 is made to becapable of detecting a force acting between the ball nut 9 and the screwpusher plate 3, that is, a force and a reaction force acting indirections opposite to each other along the direction of the screw shaftbetween the outer peripheral portion and inner peripheral portion of theload cell body 11. In effect, the thin-wall portion 11a is easilydeformed by the effect of the injection holding pressure or the backpressure acting on the screw 7, so that a resin reaction force acting onthe screw 7 can be detected.

In order to accurately detect the resin reaction force acting on thescrew 7, it is important to raise the repeatable accuracy of pressuredetection by the load cell 10. For this reason, a temperaturecompensation circuit for restraining detection errors due to the effectof ambient temperature, or coating etc. for restraining a detectionerror due to the effect of humidity are provided to the load cell 10.

However, the installation position of the load cell 10 is close to thecylinder 6, which is a heat source, and the whole of the injectionmechanism is placed in a housing of the injection unit together with themetering motor and injection motor, so that the ambient temperature ofthe load cell 10 during the actual injection molding work is oftenconsiderably higher than the ambient temperature at the time of themanufacturing the load cell 10, and the error correction by thetemperature compensation circuit alone is sometimes insufficient foraccurate measurement.

It is important to provide moisture-proof coating to the load cell 10 toprevent the adverse effect of humidity. However, if air bubbles enterthe interior of coating material or the interface with the load cell 10during the coating, the air bubbles expand or contract by thetemperature change, influencing the deformation of the load cell 10 andsometimes giving detrimental effect on the pressure detection accuracy.Needless to say, the effect of internal stress on the load cell causedby the coating the load cell 10 with a material having a differentcoefficient of thermal expansion cannot be neglected.

Further, if the coating material is subjected to plastic deformation,when a strain due to a load is produced in the load cell 10, the coatingmaterial inhibits the elastic return of the load cell 10, sometimescausing the detection pressure of the load cell 10 from returning tozero point even after the load is removed from the load cell 10.

Such a problem is not limited to the coating material. Even an adhesivefor fixing the strain gauge 12 to the load cell body 11 has the sameproblem. The problem with the adhesive is especially serious because theexpansion, contraction or plastic deformation and the like of theadhesive directly affect the strain of the strain gauge 12 itself.

In consequence, even if the preventive measures such as the temperaturecompensation circuit or moisture-proof treatment by coating are taken,it is actually impossible to make a complete compensation for the repeataccuracy of pressure detection by the load cell 10.

When only one set of the load cell 10 is installed on the powertransmitting path between the screw 7 and the injection motor, themaximum pressure which must be detected by this load cell 10 is, forexample, 2000 kg/cm², though this pressure depends on the maximuminjection holding pressure of the injection molding machine. Even if theaforesaid temperature compensation circuit or moisture-proof treatmentis applied, the detection accuracy of the load cell 10 has an error ofat least several percent finally, so that even if the error of detectionaccuracy is assumed to be 1% (actually several percent as describedabove), the final accuracy error of the load cell 10 will be about 20kg/cm² in the above case.

On the other hand, the back pressure in metering is usually set by theunit of 10 kg/cm² such as 20 kg/cm², 30 kg/cm², . . . . Therefore, if anattempt is made to carry out the back pressure control in metering byusing the same load cell 10 as that used for the detection of injectionholding pressure, the ratio of error becomes too high, so that theproper back pressure control cannot be carried out.

Thereupon, when the temperature of the load cell 10 is too high for theerror correction by the temperature compensation circuit, or when theplastic deformation and the like of the coating material affect theelastic deformation of the load cell 10, it is necessary to readjust thezero point of the load cell 10 in accordance with the error produced bythe ambient temperature, plastic deformation of coating material, etc.In this case, all disturbance elements other than the error due totemperature change or plastic deformation of coating material must beexcluded to make an accurate zero adjustment. In other words, it isnecessary to make an adjustment such that the detection pressure of theload cell 10 becomes zero in a state in which only a stress due totemperature change or plastic deformation of coating material acts.

The error caused by the plastic deformation of coating material and thelike must also be taken into account even in the case of a pressuredetector for an ejector rod or movable platen, which is locatedrelatively far away from the cylinder 6 etc., which is to be less proneto be affected by temperature. In particular, in the case of aninjection molding machine having a construction designed so that thevolume and pressure in the mold are regulated by moving an ejector pinand other movable members protruding in the mold by the ejector rod, ahigh detection accuracy is required for the pressure detector of theejector rod. Also, the detection error of the pressure detector of themovable platen, on which a great mold clamping force acts, caused by theplastic deformation of coating material and other factors, becomes aproblem, so that the similar adjustment work is sometimes needed.

A problem involved here is how to remove the effects of all theaforesaid other disturbances on the load cell 10.

The most reliable method in the case of the pressure detector of thescrew shaft in the configuration shown in FIG. 1 is to remove the bolts13, which fix the load cell 10 to the screw pusher plate 3, to retreatthe ball nut 9 and the load cell 10 to the right in the figure, or toremove the bolts 13 to advance the screw pusher plate 3 to the left inthe figure, thereby enabling the load cell 10 to be separated from othermembers and allowing the load cell 10 itself to be made completely free.

However, such work requires an extremely complicated procedure. Further,if the ball nut 9, which is separated from the screw pusher plate 3together with the load cell 10, is inadvertently rotated by hand, apositional shift as to the rotational position detector, such as a pulsecoder, installed on the ball screw 8 or the rotor shaft of injectionmotor for detecting the position of the screw 7 may produce adetrimental effect on the position control of the screw 7.

Therefore, a method is demanded in which the disturbance elements areremoved from the load cell 10 without disassembling the injectionmechanism.

However, as shown in FIG. 1, frictional forces Al and A2 such as to holdthe screw pusher plate 3 at the current position exist between the tierod 2 and the screw pusher plate 3. Specifically, there exist africtional force Al acting as a reaction force when a retreating forceacts on the screw pusher plate 3 and a frictional force A2 acting as areaction force when an advancing force acts on the screw pusher plate 3.

Also, a frictional force to hold the screw 7 at the current positionexists between the screw 7 and the injection cylinder 6. Further, whenthe resin has been solidified, a resistance to hold the screw 7 at thecurrent position exists between the resin sticking to the inside wall ofthe injection cylinder 6 and the screw 7. On the other hand, when theresin is melted, there is a case where a resin reaction force to retreatthe screw 7 exists between the screw 7 and the resin, and there isanother case in which, when the molten resin is solidifying and itsvolume is decreasing, a force to advance the screw 7 exists between thescrew 7 and the resin. In FIG. 1, for convenience, the total resinreaction force to advance the screw 7 is indicated by B1, and the totalresin reaction force to retreat the screw 7 by B2.

At a glance, it seems that, when the rotation of the ball screw 8 ismade free by cutting off the drive of the injection motor for drivingthe ball screw 8, the ball nut 9 moves back and forth freely in responseto a force such as the tension (B1-A2) acting on the load cell 10 or thestress (B2-A1), and further that the ball screw 8 side rotates freely inresponse to the longitudinal movement of the ball nut 9 to remove thetension (B1-A2), the stress (B2-A1), etc., thereby completely removingthe disturbance elements acting on the load cell 10.

Actually, however, even if the drive of the injection motor is cut off,there is still some degree of friction between the ball screw 8 and theball nut 9, and this friction acts in a direction such that the freerotation of the ball screw 8 is hindered, so that a force to keep thecurrent position of the ball nut 9 on the ball screw 8 acts. Therefore,the ball nut 9 cannot be moved back and forth freely in response to thetension or stress acting on the load cell 10, so that the disturbanceelements cannot be removed completely from the load cell 10. Needless tosay, the frictional resistance, etc. at the rotor portion of theinjection motor also present a problem.

Also, a zero adjusting method, in which a predetermined thrust force isgiven to the screw 7 to detect the reaction force by the load cell 10,and zero adjustment is made so that the given thrust force and the valuedetected by the load cell 10 agree with each other, has been disclosedin Unexamined Japanese Patent Publication No. 7-205229, and this methodhas produced satisfactory results in improving the detection accuracy ofthe injection holding pressure and the back pressure in metering. Inthis configuration, however, the load cell 10 picks up disturbance etc.due to the frictional resistance acting on various components of thedriving system in addition to the thrust force given to the screw 7, sothat the detected value including the disturbance is finally adjusted,and thus the zero adjustment of the load cell 10 alone cannot be made.

As described above, in making the zero adjustment of the load cell 10,it is too time-consuming to remove the disturbance elements byseparating the load cell 10 from other members by disassembling theinjection mechanism. Furthermore, merely de-energizing the injectionmotor for driving the ball screw 8 is not good enough to completelyremove the disturbance elements of load acting on the load cell 10. Whenthe zero adjustment of the load cell 10 is made including thedisturbance elements, it is sometimes difficult to check the abnormalityof the load cell 10 itself.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a zero adjusting methodand apparatus for a pressure detector on an injection molding machine,in which the zero adjustment of the pressure detector can be madeproperly in a state in which external loads acting on the pressuredetector are removed completely without the need of disassembling aninjection mechanism.

In the present invention, a movable member of the injection moldingmachine is driven to start reciprocating motion, in which the amplitudedecreases gradually, at an arbitrary position, and the reciprocatingmotion is stopped when the amplitude becomes a value not higher than apreset value. The output of the pressure detector at the time when thereciprocating motion is stopped is read, and the output of the pressuredetector is corrected based on the read value.

More particularly, a drive source for driving the movable member isdriven in both normal and reverse directions for causing the movablemember to make the reciprocating motion until the amplitude of themovable member becomes a value not higher than the preset value so thatthe centers of the normal and reverse drive of the drive source coincidewith the center of the reciprocating motion of the movable member. Bydoing so, a backlash in the normal and reverse directions is producedbetween the movable member and the drive source, the movable member issubstantially separated from a power transmitting path, and the zeroadjustment of the pressure detector is made in a state in which externalloads acting on the movable member are removed completely.

In starting the reciprocating motion, therefore, it is more effective tomake coincide the center of reciprocating motion with the center ofbacklash in advance. This is because the range itself of thereciprocating motion of the movable member is sometimes limited for somereason like the case of a screw shaft etc. whose movement is limited bysolidified resin.

Thus, in order to serve this purpose, the output of a drive source fordriving the movable member is made low, and a command for moving themovable member by a preset amount is issued to the drive source withreference to the current position of the movable member. Then, the stopposition of the movable member is detected and the stop position isstored as a first stop position.

Further, a command for moving the movable member in the reversedirection by the preset amount is issued to the drive source withreference to the current position of the movable member. Then, the stopposition of the movable member is detected and stored as a second stopposition. And, the reciprocating motion is performed with respect to oneintermediate point between the first and second stop positions being thecenter of the movement.

In a situation in which the movement itself of the movable member iscompletely fixed, it is meaningless to reciprocate the movable memberitself. Therefore, the reciprocating motion is performed by taking, asan initial value of amplitude, a value which is not higher than 1/2 ofthe distance between the first and second stop positions and not lowerthan 1/2 of the amount of backlash produced between the movable memberand drive source.

On the other hand, when the zero adjustment of the pressure detector forscrew shaft has to be made with the melted resin remaining in acylinder, or when the zero adjustment of the pressure detector providedfor the movable member having an adherent sliding resistance to a guiderod etc., has to be made, not only the connecting portion of the movablemember and the power transmitting path but also the pressure, viscosity,etc. of resin acting on the movable member can affect the pressuredetector depending on the situation. Therefore, by allowing thereciprocating motion to be performed by taking, as an initial value ofamplitude, a value which is obtained by adding a preset value to a valueof 1/2 of the distance between the first and second stop positions, or avalue, which is obtained by multiplying a value of 1/2 of the distancebetween the first and second stop positions by a number higher than 1,thereby enabling the movable member to move positively at the initialstage of reciprocating motion so that the resin, the oil etc. at thesliding portion is made to move more easily and work more effectivelyfor the movable member, by which the external forces due to the pressureand viscosity of resin and the viscosity etc. of oil at the slidingportion are removed from the movable members.

Designed so that a backlash in the normal and reverse directions isproduced between the movable member and drive source to substantiallyseparate the movable member from the power transmitting path and removethe external loads acting on the movable member, it is desirable finallyto continue the reciprocating motion until the amplitude of drive sourcebecomes smaller than 1/2 of the amount of overall backlash occurringbetween the movable member and the drive source, that is, until theconnection between the movable member and the power transmitting path issevered.

The drive source for driving the movable member can be composed of aservomotor having output limiting means.

In the case of the pressure detector for detecting the pressure actingon the screw shaft, both the automatic purging operation and the zeroadjustment of the aforesaid pressure detector can be carried outautomatically, so that the zero adjustment of the pressure detector canbe made without fail at proper time intervals such as those for thechange of material for molding.

Also, an abnormality detection signal is generated when the absolutevalue of correction amount of the output of pressure detector exceeds apredetermined amount, thereby enabling serious abnormality, which cannotbe corrected by zero adjustment only, to be detected.

Further, the history of zero adjustment including the date andcorrection amount of adjustment is stored in a control unit of theinjection molding machine and the stored history is displayed, wherebythe deterioration in performance or the presence of serious abnormalityof the pressure detector can be found more easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a principal portion of a pressure detectorhaving a typical construction using a load cell;

FIG. 2 is a block diagram showing a principal portion of a controllerfor driving and controlling the various parts of an injection moldingmachine;

FIG. 3 is a schematic diagram showing screw positions in accordance withthe procedure of zero adjustment, with the tip end of an injectioncylinder being an origin;

FIG. 4 is a flowchart showing zero adjustment procedure for automaticzero adjustment;

FIG. 5 is a continued flowchart showing the zero adjustment procedure;

FIG. 6 is a continued flowchart showing the zero adjustment procedure;

FIG. 7 is a continued flowchart showing the zero adjustment procedure;and

FIG. 8 is a diagram conceptually showing a file for storing theadjustment history.

BEST MODE OF CARRYING OUT THE INVENTION

The principle of the present invention will be described with referenceto FIG. 1, for a pressure detector for detecting a pressure acting on ascrew as an example.

First, a command for moving a screw 7 by a preset amount with referenceto the present screw position is issued to a drive source so that thescrew 7 is moved back and forth with a small force (low output) untilthe movable limit position is reached.

In such a condition, if the resin in an injection cylinder 6 is melted,even in a small amount, and in a state such that it can be compressed ineither direction, the drive source can operate exceeding at least thebacklash amount, so that the screw 7 can be moved back and forth to someextent within the range of the aforesaid preset amount.

More strictly speaking, when the resin in the injection cylinder 6 ismelted to a degree at which its viscosity has become considerably low ascompared to the viscosity at room temperature, the screw 7 can advanceor retreat by the aforesaid preset amount from the initial screwposition being the reference. If the melting of resin is insufficient,and thus the viscosity thereof remains considerably high, it isimpossible for the drive source to move the screw 7 by the aforesaidpreset amount with the aforesaid small force. Therefore, the screw 7advances or retreats by an amount less than the aforesaid preset amountfrom the initial screw position being the reference.

When the viscosity of the resin is low, that is, when the screw 7 canmove by the aforesaid preset amount, a resin reaction force will notcause a big problem as a disturbance element. The resin reaction forcewill become a problem as a disturbance element in such a case where theviscosity of resin is high, and a resin reaction force (B2) to retreatthe screw 7 acts between the screw 7 and the resin in a state in whichthe screw 7 stops at the initial screw position, for example, in a casewhere the screw 7 is stopped at the initial screw position in a state inwhich a positive compressive force is left in the resin by discontinuingthe advancing motion of the screw 7, and in such a case where a resinreaction force (B1) to advance the screw 7 acts between the screw 7 andthe resin in a state in which the screw 7 stops at the initial screwposition, for example, in a case where the screw 7 is stopped at theinitial screw position in a state in which a negative compressive forceis left in the resin by discontinuing the retreating motion of the screw7, and in the like cases.

As an example, consideration is given to the case where the resinreaction force (B2) to retreat the screw 7 acts between the screw 7 andthe resin in a state in which the screw 7 stops at the initial screwposition.

As described above, if the screw 7 is advanced with a small force fromthe initial screw position being the reference, when the resin reactionforce (B2) to retreat the screw 7, which increases in proportion to thecubic compression of resin due to the advance of the screw 7 balanceswith the aforesaid small force of the drive source to advance the screw7, the advance of the screw 7 stops. This position is stored as thefirst stop position.

If the screw 7 is retreated with a small force from the initial screwposition being the reference, when the resin reaction force (B1) toadvance the screw 7, which increases in proportion to the volumeexpansion of resin due to the retreat of the screw 7, balances with theaforesaid small force of the drive source to retreat the screw 7, theretreat of the screw 7 stops. This position is stored as the second stopposition.

The position of the screw 7, at which the resin reaction force (B1, B2)acting on the screw 7 becomes substantially zero, is the middle pointbetween the limit position, to which the screw 7 advances when the screw7 is advanced with a predetermined force, and the limit position, towhich the screw 7 retreats when the screw 7 is inversely pulled with thesame force as the aforesaid predetermined force, that is, theintermediate position between the first and second stop positions.Thereupon, this intermediate position is first determined, and the screw7 is moved to this position, by which the effect of the resin reactionforce on the screw 7 is removed.

In consequence, when the resin reaction force (B2) to retreat the screw7 acts between the screw 7 and the resin in a state in which the screw 7stops at the initial screw position, the aforesaid intermediate positionshifts backward to the rear with respect to the initial screw position.When the resin reaction force (B1) to advance the screw 7 acts betweenthe screw 7 and the resin in a state in which the screw 7 stops at theinitial screw position, the aforesaid intermediate position shiftsforward with respect to the initial screw position.

When the viscosity of resin is low, that is, when the screw 7 can bemoved freely by the aforesaid preset amount, the advance limit positionof the screw 7 shifts forward by the aforesaid preset amount withrespect to the initial screw position, and the retreat limit position ofthe screw 7 shifts backward by the aforesaid preset amount with respectto the initial screw position. In consequence, the calculatedintermediate position agrees with the initial screw position. As alreadydescribed, in such a case, the resin reaction force will not become abig problem as a disturbance element.

However, in order to decrease the effect of the resin reaction force, asdescribed later specifically, the screw is reciprocated (or vibrated)with respect to one point of the intermediate portion being the center,by which a force exerted from the resin on the screw at the intermediateposition is made to be eliminated.

If the screw is reciprocated until the reciprocating stroke of the drivesource for driving the screw becomes smaller than the backlash, thescrew stops to move further, and only the drive source reciprocates. Ifthe screw is stopped at this stage, the screw will not have any forceacting thereon from either the resin or the drive source, so that nopressure is applied to the pressure detector.

On the other hand, if the resin in the injection cylinder 6 is notmelted at all, and the screw 7 is completely at rest with respect to theinjection cylinder 6, the drive source cannot be operated for themovement exceeding the backlash amount, and naturally the screw 7 willnot move.

Thus, the first stop position in this case is a position of the drivesource at which the drive source operates in the direction for advancingthe screw 7 and for eliminating a backlash between the screw 7 and thedrive source, thereby allowing the driving force of the drive source tobe transmitted directly to the screw 7 as an advance force, while thesecond stop position is a position of the drive source at which thedrive source operates in the direction for retreating the screw 7 andfor eliminating the backlash between the screw 7 and the drive source sothat the driving force of the drive source is transmitted directly tothe screw 7 as a retreating force.

Thus, when the resin in the injection cylinder 6 is not melted at all,and the screw 7 is completely at rest with respect to the injectioncylinder 6, the drive source is moved to the intermediate position ofbacklash, that is, the position at which neither the driving force ofthe drive source is transmitted directly to the screw 7 as an advancingforce nor the driving force of the drive source is transmitted directlyto the screw 7 as a retreating force, by a command for the movement tothe intermediate position. At a glance, it seems that, under such acondition, the connection between the screw 7 and the drive source iscompletely cut off, and the external forces acting on the pressuredetector 10 are removed completely.

When the power transmitting path from the drive source to the screw 7 iscomplex, however, a backlash is produced with each of the components ofthe power transmitting path. Even if the drive source is moved to theintermediate position of the overall backlash between the screw 7 andthe drive source, all the backlashes of the components are not alwaysshifted to the respective intermediate positions.

That is, just like the above-described case where the screw 7 is movedfrom the second stop position, which is the retreat limit position, tothe intermediate position in a state in which the resin is melted tosome extent, there is a possibility that the pressure detector 10between a screw pusher plate 3 and the ball nut 9 is under the influenceof some external force acting thereon.

For example, in the example shown in FIG. 1, if the magnitude ofbacklash between the ball nut 9 and the ball screw 8, the magnitude ofbacklash between the ball screw 8 and a timing belt for driving the ballscrew 8, and the magnitude of backlash between the timing belt and aninjection motor for driving the timing belt are taken as C,respectively, the amount of backlash of the whole power transmittingpath from the drive source to the pressure detector 10 is 3C.

Then, consideration is given to the case where the position at which thebacklash in the advancing direction of the screw 7 is eliminated, thatis, the position at which the driving force of the injection motor istransmitted directly to the screw 7 as an advancing force is called afirst stop position, and the position at which the backlash in theretreating direction of the screw 7 is eliminated, that is, the positionat which the driving force of the injection motor is transmitteddirectly to the screw 7 as a retreating force is called a second stopposition; the movement is made from the first stop position to thesecond stop position. The injection motor is rotated by (3/2)·C from thesecond stop position. The injection motor is them moved to the middlepoint between the first stop position and the second stop position, thatis, the intermediate position of the overall backlash.

In other words, this is a movement from the second position, at whichthe backlash in the retreating direction of the screw 7 is absent, andthe backlash in the advancing direction of the screw 7 is at a maximum,to the intermediate position.

Naturally, the order in which the backlash in the advancing direction ofthe screw 7 is eliminated by the rotation of the injection motor in thiscase, when a set of components of each part is viewed as a unit, thefirst is the backlash between the timing belt for driving the ball screw8 and the injection motor, the next is the backlash between the ballscrew 8 and the timing belt, and the last is the backlash between theball nut 9 and the ball screw 8.

At this time, since the travel amount (3/2)·C of the injection motor islarger than the backlash C between the timing belt for driving the ballscrew 8 and the injection motor, the backlash in the advancing directionof the screw 7 between the timing belt and the injection motor iseliminated completely when the injection motor moves by C. As a result,the timing belt is moved in the advance direction of the screw 7 by[(3/2)·C-C=(1/2)·C], and at the same time, the injection motor completesthe commanded travel of (3/2)·C and stops.

However, since the rotation amount (3/2)·C of the timing belt is lessthan the backlash amount C between the ball screw 8 and the timing belt,the backlash in the advance direction of the screw 7 between the ballscrew 8 and the timing belt is not eliminated completely, so that theball 8 itself does not rotate at all.

Thereupon, the relationship between the ball nut 9 and the ball screw 8is held in a state in which the injection motor is rotated from thefirst stop position toward the second stop position, that is, a state inwhich the backlash in the retreat direction of the screw 7 is eliminatedcompletely, by which the ball nut 9 is pulled by the ball screw 8.Therefore, the tension acting on the pressure detector 10 cannot beremoved.

This means that even if the injection motor is rotated in the reversedirection from the first stop position toward the second stop position,and then rotated in the normal direction by 1/2 of the overall backlashamount to be moved to the middle point between the first and second stoppositions, the tension between the ball screw 8 and the ball nut 9cannot always be removed.

As far as the above-described example in which the magnitude of backlashbetween the ball nut 9 and the ball screw 8, the magnitude of backlashbetween the ball screw 8 and the timing belt, and the magnitude ofbacklash between the timing belt and the injection motor are taken as C,respectively is concerned, when the magnitude C of backlash between theball screw 8 and the timing belt is added to the magnitude C of backlashbetween the timing belt and the injection motor, and the sum is furtheradded to the half of magnitude (1/2)·C of backlash between the ball nut9 and the ball screw 8 to obtain a value of (5/2)·C, and the injectionmotor is rotated in the normal direction by this (5/2)·C to be movedfrom the second stop position toward the first stop position, the stressor tension directly affecting the pressure detector 10 can be removed.

Actually, however, since the backlash amount of each part of the powertransmitting path varies depending on the degree of wear of the machine,etc., the travel amount of the drive source to move the backlash betweenthe ball nut 9 and the ball screw 8 to the neutral position cannot bedetermined on a single basis such as a predetermined value etc.Therefore, it is necessary to perform a processing based on a certainalgorithm designed for moving the backlash between the ball nut 9 andthe ball screw 8 to the neutral position.

The precedence of advance or retreat of the screw 7 may be determinedarbitrary. Inverse to the above description, when the retreat operationof the screw 7 is performed first, the retreat stop position is storedas the first stop position, and the advance stop position is stored asthe second stop position, but the calculation result regarding theintermediate position is the same.

In order to fulfill the task of the present invention, with one point ofthe intermediate portion serving as a center, a value which is nothigher than 1/2 of the distance between the first and second stoppositions and not smaller than 1/2 of the total amount of backlashproduced between the screw and the drive source is taken as the initialvalue of amplitude, and the drive source is reciprocated at least untilthe amplitude becomes smaller than 1/2 of the backlash amount. Thephenomenon produced by this algorithm will be explained below based onthe typical configuration shown in FIG. 1.

First, the magnitude of amplitude at the initial stage of startedreciprocation is 1/2 of the distance between the first and second stoppositions, that is, 1/2 of the magnitude of the overall backlash, whichis equal to or higher than (3·C)/2, for example. This value is largerthan the sum of the magnitude of backlash between the ball nut 9 and theball screw 8, the magnitude of backlash between the ball screw 8 and thetiming belt, and the magnitude of backlash between the timing belt andthe injection motor. Therefore, the ball screw 8 rotates repeatedly inthe normal and reverse directions, exceeding an interval between aposition, at which the backlash in the advance direction of the screw 7between the ball nut 9 and the ball screw 8 is eliminated and the normalrotation of the ball screw 8 is directly transmitted to the ball nut 9as the advance force of the screw 7, and a position, at which thebacklash in the retreat direction of the screw 7 between the ball nut 9and the ball screw 8 is eliminated and the normal rotation of the ballscrew 8 is directly transmitted to the ball nut 9 as the retreat forceof the screw 7.

When the magnitude of amplitude decreases gradually to a magnitudesmaller than 1/2 of the overall backlash amount, for example, (3·C)/2,the ball screw becomes unable to rotate to the position at which thenormal rotation of the ball screw 8 is directly transmitted to the ballnut 9 as the advance force of the screw 7 or to the position at whichthe reverse rotation of the ball screw 8 is directly transmitted to theball nut 9 as the retreat force of the screw 7, so that the substantialcontact between the ball screw 8 and the ball nut 9 is cut off.

In this state, since the ball screw 8 is completely separated from theball nut 9, that is, they are not in contact with each other, neither aforce of the ball nut 9 to push the pressure detector 10 nor a force ofthe ball nut 9 to pull the pressure detector 10 will act, so that thedisturbance to act on the pressure detector 10 substantially becomeszero.

Needless to say, even in such a state, if there is a possibility that aresin pressure etc. acts on the screw 7 to produce a force to push orpull the screw pusher plate 3, causing the screw pusher plate 3 itselfto be moved, this possibility cannot be neglected. However, as describedabove, in the present invention, the resin reaction force itself actingon the screw 7 has been removed in advance by the movement of the drivesource from the second stop position to the intermediate position, sothat there is no problem in this respect.

Thereupon, as shown in FIG. 1, if the pressure detector 10 is disposedat a final position on the power transmitting path, for example, at aposition between the ball nut 9 and the screw pusher plate 3, when themagnitude of amplitude becomes smaller than 1/2 of the overall backlashamount, (3·C)/2, the reciprocating motion can be stopped immediately tomake the zero adjustment of the pressure detector 10. Naturally, theexternal loads acting on the pressure detector 10 in this state arezero.

If the reciprocating motion is to be continued without stopping evenwhen the magnitude of amplitude becomes smaller than 1/2 of the overallbacklash amount, (3·C)/2, in the above example, the normal and reverserotation of the ball screw 8 stops completely when the amplitudedecreases to C, that is, when the amplitude becomes smaller than the sumof the magnitude of backlash between the ball screw 8 and the timingbelt and the magnitude of backlash between the timing belt and theinjection motor. This is because the reciprocating motion of the drivesource is absorbed completely by the backlash between the ball screw 8and the timing belt and the backlash between the timing belt and theinjection motor, causing the driving force of the drive source is not tobe transmitted to the ball screw 8. The position at which the normal andreverse rotation of the ball screw 8 stops is naturally the center ofthe reciprocating motion of the ball screw 8, which lies at anintermediate point between the position at which the normal rotation ofthe ball screw 8 is directly transmitted to the ball nut 9 as theadvance force of the screw 7 and the position at which the reverserotation of the ball screw 8 is directly transmitted to the ball nut 9as the retreat force of the screw 7.

If the reciprocating motion is to be continued further under the samecondition, in the above example, the normal and reverse rotation of thetiming belt stops completely when the amplitude has decreased to C/2,that is, when the amplitude has become smaller than the magnitude ofbacklash between the timing belt and the injection motor. This isbecause the reciprocating motion is absorbed completely by the backlashbetween the timing belt and the injection motor, causing the drivingforce of the drive source not to be transmitted to the timing belt. Theposition at which the normal and reverse rotation of the timing beltstop is naturally the center of the reciprocating motion of the timingbelt, which lies at an intermediate point between a position at whichthe normal rotation of the timing belt is directly transmitted to theball nut 8 as a normal rotational force of the ball nut 8 and a positionat which the reverse rotation of the timing belt is directly transmittedto the ball nut 8 as the reverse rotational force of the ball nut 8.

Finally, the position of injection motor at which the amplitude ofinjection motor decreases to zero to come to rest is the center of thereciprocating motion of the injection motor itself, which lies at anintermediate point between a position at which the normal rotation ofthe injection motor is directly transmitted to the timing belt as thenormal rotational force of the timing belt and a position at which thereverse rotation of the injection motor is directly transmitted to thetiming belt as the reverse rotational force of the timing belt.

That is, according to the present invention, by carrying out thereciprocating motion until the magnitude of amplitude finally convergesto zero or to a value approximate to zero, all the backlashes of thecomponents can be shifted to the neutral position of each backlash.

In the above-described example shown in FIG. 1, it cannot be thought byany means that the pressure detector 10 is installed at a position otherthan that between the ball nut 9 and the screw pusher plate 3, forexample, at a position between the ball screw 8 and the timing belt fordriving the ball screw 8 or between the timing belt and the injectionmotor for driving the timing belt. In an injection mechanism configuredby a power transmitting path consisting of components different fromthose in the above-described example, however, the pressure detector 10sometimes can be disposed at a position on the power transmitting pathdifferent from the position between the ball nut 9 and the screw pusherplate 3 in the example shown in FIG. 1.

According to the present invention utilizing reciprocating motion, eachof all the backlashes of the components can be shifted finally to theneutral position of each backlash regardless of the configuration ofpower transmitting path. Thus, regardless of the position ofinstallation of the pressure detector 10, a substantial connectionbetween a component, which gives a direct load to the pressure detector10, and the pressure detector 10 can be cut off, whereby the disturbanceto the pressure detector 10 can be removed.

When the resin in the injection cylinder 6 is melted, the initial valueof the amplitude is larger than the sum of 1/2's of backlash amount ofmechanical system. In this case, however, the movement itself of thescrew 7 is allowed, so that there is no problem. In such a case, themovement itself of the screw 7 can be thought as a part of the overallbacklash, and the screw 7 itself stops at the center of thereciprocating motion, that is, the aforesaid intermediate position, likethe cases of aforesaid components.

The final zero adjustment is made by reading the present value of thepressure detector, that is, the output of the pressure detector in theno-load condition after the completion of reciprocating motion, and byadjusting the output of the pressure detector so that the present valuebecomes zero.

Also, as a means for moving the screw 7 in a state in which the outputof the drive source for driving the screw 7 is low, a servomotor havingoutput limiting means can be used.

Further, by applying a value obtained by adding a preset value to 1/2 ofthe distance between the first and second stop positions, or a valueobtained by multiplying 1/2 of the distance between the first and secondstop positions by a number larger than 1 as the initial value of theamplitude, even if the backlash is increased by the wear of the machine,proper measures can be taken against such a situation.

Basically the same procedure as that described above can be applied tothe case of pressure detector for an ejector rod or a movable platentoo.

FIG. 2 is a block diagram showing a principal portion of a numericalcontrol unit 100 used as a controller for controlling the drive of partsof the injection molding machine.

The numerical control unit 100, having a CPU 115 for CNC, which is amicroprocessor for numerical control, a CPU 108 for PMC, which is amicroprocessor for programmable machine controller, a servo CPU 110,which is a microprocessor for servo control, and a CPU 107 for pressuremonitor, which performs sampling by detecting the injection holdingpressure and the back pressure during metering from the load cell 10 onthe injection molding machine side via an A/D converter 106, cantransmit information between the microprocessors via a bus 112 byselecting input and output.

Connected to the CPU 108 for PMC are a ROM 103, which stores a sequenceprogram for controlling the sequence operation of the injection moldingmachine and other programs, and a RAM 104, which is used for temporarystorage of arithmetic data and for other purposes. Connected to the CPU115 for CNC are a ROM 117, which stores a control program forcontrolling the drive of each axis of the injection molding machine andother programs, and a RAM 118, which is used for temporary storage ofarithmetic data and for other purposes.

Also, connected to the servo CPU 110 are a ROM 111, which stores acontrol program dedicated for servo control, and a RAM 109, which isused for temporary storage of data. Connected to the CPU 107 formonitoring pressure are a ROM 101, which stores a control programregarding the sampling of injection holding pressure and back pressureand other processing, and a RAM 102, which is used for temporary storageof data. The RAM 102, which is a nonvolatile memory, has a storageregion for storing the correction value of the load cell 10. The CPU 107for monitoring pressure detects a value which is obtained by adding acorrection value to the output of the A/D converter 106 as an injectionholding pressure and back pressure. However, when the later-describedzero adjustment is made, the aforesaid addition is canceled, and theoutput of the A/D converter 106 is read directly.

A servo-amplifier 105, which drives servomotors of axes for moldclamping, ejector, injection, screw rotation, etc. based on the commandfrom the CPU 110, is connected to the servo CPU 110 so that the outputof a pulse coder attached to the servomotor of each axis is fed back tothe servo CPU 110. The current position of each axis is calculated bythe servo CPU 110 based on the feedback pulse from the pulse coder, andis updated and stored in a current position storage register of eachaxis.

FIG. 2 shows only a servomotor M1 for injection, which drives theservo-amplifier 105 for one axis and the ball screw 8 of the injectionmechanism, and a corresponding pulse coder P1. The configuration of eachaxis for mold clamping, ejector, sprue break, etc. is the same as theabove configuration. However, the configuration for screw rotation neednot detect the current position, and only has to detect the speed.

An interface 113 is an input/output interface for receiving signals fromlimit switches installed on various components of the injection moldingmachine body and a control panel and for transmitting various commandsto peripheral equipment of the injection molding machine.

A manual data input device 119 with display is connected to the bus 112via a CRT display circuit 116 so that the graphic display and functionmenu on the screen can be selected, and the input operation of variousdata, etc. can be performed. Further, this device is provided with tenkeys for inputting numerical data, various function keys, etc.

A nonvolatile memory 114 preserves molding data such as the moldingconditions for injection molding work and various setting values,parameters, macro variables, etc.

By the above-described configuration, the CPU 108 for PMC controls thesequential operation of the whole injection molding machine, the CPU 115for CNC distributes pulses to the servomotors of axes based on theoperation program for each axis in ROM 117, the molding conditions inthe nonvolatile memory 114, etc.; and the servo CPU 110 carries outservo control such as position loop control, speed loop control, andcurrent loop control based on a movement command distributed by pulse toeach axis and a feedback signal of the position and speed detected by adetector such as the pulse coder, as in the conventional case. Thus,so-called digital servo processing is executed for injection moldingwork.

FIGS. 4 to 7 are flowcharts schematically illustrating zero adjustmentprocessing for automatically making zero adjustment of the load cell 10.The program for this zero adjustment processing is executed by the CPU115 for CNC by selecting "zero adjustment of load cell" from thefunction menu.

The CPU 115 for CNC, which has started the zero adjustment processingfirst reads the current position B of the motor for sprue break (StepS1), and determines whether or not the injection unit has retreated to apreset retreat position for purging (Step S2). If the injection unit hasnot retreated to the preset retreat position, a retreat command isissued to the motor for sprue break (Step S3) to make the injection unitretreat to the preset retreat position for purging (Step S4), and stopat the preset retreat position (Step S5).

This process is designed to prevent molten resin from being dischargedand sticking to the mold and stationary platen when the screw 7 isadvanced. Therefore, when the injection unit has already retreated tothe preset retreat position for purging as in the case where automaticpurging has been executed, that is, when the result of determination inStep S2 is Yes, naturally this processing will not be executed.

Next, the CPU 115 for CNC initially sets zero, the value indicatingmelting, in a flag F which stores the solidified state of resin (StepS6), and reads and stores the current position Ps of the servomotor M1for injection, that is, the current position of the screw 7 (Step S7).This value Ps is the aforesaid initial current position of the screw.However, as described previously, since there is some backlash betweenthe servomotor M1 for injection and the screw 7, sometimes an erroroccurs, within the range corresponding to the backlash, between thecurrent screw position corresponding to the position of the servomotorM1 for injection and the actual current screw position.

For example, in the case where the screw 7 is stopped at the initialcurrent position of screw in a state such that a positive compressiveforce is left in the resin by interrupting the advancing or the screw 7,the actual current screw position is sometimes slightly behind thecurrent screw position corresponding to the position of the servomotorM1 for injection. On the other hand, in the case where the screw 7 isstopped at the initial current position of screw in a state such that anegative compressive force is left in the resin by interrupting theretreating of the screw 7, the actual current screw position issometimes slightly ahead of the current screw position corresponding tothe position of the servomotor M1 for injection.

One example of the initial current position Ps of screw is shown on thenumeric straight line in FIG. 3, which shows the position of screw 7with the front end of the injection cylinder 6 being the origin.

Then, the CPU 115 for CNC, which has stored the initial current positionPs of the servomotor M1 for injection sets a torque limit T1 in theservo CPU 110 to limit the driving force of the servomotor M1 forinjection to a low output (Step S8), initially sets zero in a counter Cfor counting the number of reciprocating motions of the screw 7 (StepS9), increases the value of the counter C by one increment (Step S10),and outputs a movement command, which is necessary to move the screw 7from the initial current position Ps of the screw to the front end ofthe injection cylinder 6 (hereinafter referred to as the origin), to theservomotor M1 for injection (Step S11, movement to al shown in FIG. 3).

Next, the CPU 115 for CNC determines whether or not the value of thecounter C is 1, that is, whether or not the command for movement toorigin of this time is the command for the movement to origin in thefirst reciprocating motion (Step S12), and, if it is the first commandfor the movement to origin, sets a predetermined operation time in atimer T to start time counting (Step S13). This operation time has to bea time period during which the screw 7 can reach the origin even ifmolten resin having a high viscosity remains within the injectioncylinder 6.

Subsequently, the CPU 115 for CNC repeatedly executes the determinationprocessings in Steps S14 to S16. More particularly, the CPU 115 for CNCdetermines whether or not the screw 7 reaches the origin before theoperation time of timer T elapses, that is, whether or not the resin inthe injection cylinder 6 is melted and the movement of screw 7 ispossible (including the case where resin is absent in the injectioncylinder 6), or whether or not the resin in the injection cylinder 6 issolidified completely and the movement of the screw 7 is impossible(Steps S14 to S16).

Instead of determining whether or not the resin is melted in Steps S14to S16, the temperature of the injection cylinder 6 may be detected todetermine whether the temperature thereof is a temperature at which theresin is melted.

First, when the resin in the injection cylinder 6 is melted and themovement of the screw 7 is possible (including the case where resin isabsent in the injection cylinder 6), that is, when the screw 7 reachesthe origin before the operation time of timer T elapses and the decisionin Step S15 is Yes, the CPU 115 for CNC succeedingly outputs a movementcommand which is necessary to move the screw 7 from the origin to theretreat limit (Step S17, movement of a2 shown in FIG. 3), verifies themovement to the retreat limit (Step S18), again outputs a movementcommand to move the screw 7 from the retreat limit to the initialcurrent position Ps of screw (Step S19, movement of a3 shown in FIG. 3),verifies the movement to the initial current position Ps of screw (StepS20), and determines whether or not the value of the counter C forcounting the number of reciprocating motion of the screw 7 has reached apredetermined number of reciprocating operations Cx (Step S21).

If the predetermined number of reciprocating motions Cx is set at avalue not lower than 2, the screw 7 can be reciprocated in the injectioncylinder 6 a plurality of number of times with full stroke by therepeated processing in the above-mentioned Steps S10 to S21, so that aconsiderable amount of melted resin in the injection cylinder 6 can bedischarged (in the case where resin is present in the injection cylinder6). However, since the value of counter C is 2 or higher in thereciprocating operation for the second time and there after, theprocessings in Steps S13 and S14, which are necessary to determinewhether or not the movement of the screw 7 is possible, are notexecuted. If the first movement to the origin is possible, it isapparent that the resin is melted (including the case where resin isabsent in the injection cylinder 6), so that the processings in StepsS13 and S14 need not be repeated.

On the other hand, when the solidification of resin (or very highviscosity) is found in the first movement to the origin, that is, whenthe screw 7 has not reached the origin even if the operation time oftimer T has elapsed, and the decision in Step 14 is Yes, the screw 7cannot be moved or can be moved only slightly, so that it is meaninglessto output a screw retreat command. In this case, therefore, the CPU 115for CNC omits the processing for both the screw movement to the originduring execution and the screw movement to the retreat limit, outputs acommand for movement to the initial current position Ps of screw,verifies the screw movement to the initial current position Ps of screw(Steps S48 and S49), and sets 1 in flag F provided for storing thesolidified state of resin, thereby storing that the resin has beensolidified completely (Step S50).

After the CPU 115 for CNC makes the screw 7 perform reciprocating motionand return movement to the initial current position Ps (in the casewhere the resin melts and the processing on a path passing theconnecting point 2 has been performed) or makes the screw 7 return tothe initial current position Ps by omitting the movement to the origin(in the case where the resin solidifies and the processing on a pathpassing the connecting point 1 has been performed) depending on themelting state of resin, the CPU 115 for CNC sets torque limit T2 in theservo CPU 110 to limit the driving force of the servomotor M1 forinjection to a low output (Step S22).

However, the aforesaid torque limit T1 is a force of such a degree thatthe screw 7 or the driving system thereof is not damaged even if amovement command is output when the resin is in solid state and that thescrew 7 can be moved with full stroke even when the melted resin remainsin the injection cylinder 6, while the torque limit T2 is a force ofsuch a degree as to be necessary to verify the reaction force such asviscosity resistance of resin, so that the relationship of T1>T2 holds.

The CPU 115 for CNC, which has set the torque limit T2, outputs amovement command to advance the screw 7 from the initial currentposition Ps of screw by a preset amount S (Step S23, movement of a4shown in FIG. 3), sets a predetermined operation time in the timer T tolet the timer start time counting (Step S24), waits until the operationof timer T is finished (Step S25), and reads the current screw positionQf and stores it as the first stop position after the predeterminedoperation time has elapsed (Step S26).

When the resin in the injection cylinder 6 melts considerably or whenthe resin is absent in the injection cylinder 6, the screw 7 can advanceby the amount S of movement command by overcoming the resistance ofresin (the position to be reached by the movement in this case isindicated by Pf in FIG. 3). However, when the resin in the injectioncylinder 6 melts insufficiently and the resin reaction force and theviscosity resistance are high, the screw 7 cannot advance by the amountS of movement command by overcoming the resistance of the resin. In sucha case, the advance of the screw 7, is stopped at a position where theresin reaction force, viscosity resistance, etc. acting to retreat thescrew 7, which increases in proportion to the volume contraction ofresin caused by the advance of the screw 7, balance with a drivingtorque to advance the screw 7 (an example of position to be reached inthis case is indicated by Qf in FIG. 3).

When the resin in the injection cylinder 6 has solidified completely,the screw 7 itself will not move substantially, and only the servomotorM1 for injection can rotate within the allowable range of backlash.

In any case, the position detected as Qf is not the actual position ofthe screw 7 itself, but the rotational position of the servomotor M1 forinjection.

The CPU 115 for CNC, which has thus detected the first stop position,makes the screw 7 return once to the initial current position Ps ofscrew to equalize the conditions at which the movement command is issued(Steps S27 and S28, movement of a5 shown in FIG. 3), outputs a movementcommand to retreat the screw 7 from the initial current position Ps ofscrew by a preset amount S (Step S29, movement of a6 shown in FIG. 3),sets a predetermined operation time in the timer T to start timecounting (Step S30), waits until the operation of timer T is finished(Step S31), and reads the current screw position Qb and stores it as thesecond stop position after the predetermined operation time has elapsed(Step S32).

As described previously, when the resin in the injection cylinder 6melts considerably or when the resin is absent in the injection cylinder6, the screw 7 can retreat by the amount S of the movement command byovercoming the negative resistance of resin acting in the advancingdirection of the screw 7 (the position to be reached in this case isindicated by Pb in FIG. 3). However, when the resin in the injectioncylinder 6 melts insufficiently, the screw 7 cannot retreat by theamount S of the movement command by overcoming the negative resistanceof the resin. In such a case, the retreat of the screw 7 is stopped at aposition where the resin reaction force, viscosity resistance, etc.,which induces the screw 7 to advance, increase in proportion to thecubic expansion of resin caused by the retreat of the screw 7, balancewith a driving torque to cause the screw 7 to retreat (an example of aposition to be reached in this case is indicated by Qb in FIG. 3).

When the resin in the injection cylinder 6 has solidified completely,the screw 7 itself will not move substantially, and only the servomotorM1 for injection rotates within the allowable range of backlash.

In any case, the position detected as Qb is not the actual position ofthe screw 7 itself, but the rotational position of the servomotor M1 forinjection.

Thus, the CPU 115 for CNC, which has detected the first stop position Qfand the second stop position Qb as described previously, carries out theprocessing to move the actual position of the screw 7 to a position atwhich the resin reaction force balances, or the processing to move therotational position of the servomotor M1 for injection to anintermediate position of backlash.

The CPU 115 for CNC first divides the sum of Qf and Qb by 2 to determinethe rotational position Pm of the servomotor M1 for injectioncorresponding to the actual position of the screw 7 at which the resinreaction force balances (Step S33).

That is, assuming that a backlash of Da is present between the screw 7and the servomotor M1 for injection, the value of the first stopposition Qf of the screw 7 on the number line is not actually the valueof Qf but the value of Qf+Da (the backlash amount Da on the number lineacts in the positive direction because the movement from Ps to Qf is amovement in the negative direction). Also, the value of the second stopposition Qb of the screw 7 on the number line is not actually the valueof Qb but the value of Qb-Da (the backlash amount Da on the number lineacts in the negative direction because the movement from Ps to Qb is amovement in the positive direction). In order to determine the actualposition of the screw 7 at which the resin reaction force acting on thescrew 7 balances, it is necessary to take the mean value of the truepositions of Qf and Qb. As described above, the backlash Da acts in thedirections in which it is canceled with each other, so that the backlashamount Da need not be considered in the actual calculation.

In effect, [(Qf+Da)+(Qb-Da)]/2 and (Qf+Qb)/2 are consequently the same,so that the true position of the screw 7 at which the resin reactionforce balances can be determined by (Qf+Qb)/2 from the reading based onthe rotational position of the servomotor M1 for injection.

Next, the CPU 115 for CNC outputs the determined rotational position Pmas a movement command to rotate the servomotor M1 for injection and tomove the screw 7 to the intermediate position Pm (Steps S34 and S35).

According to the above processing, even if the resin melts and any resinreaction force acts, the screw 7 can be moved to a position at which theresin reaction force balances to remove the resin reaction force, andthe servomotor M1 for injection can be moved to the intermediateposition of overall backlash in the power transmitting system betweenthe screw 7 and the servomotor M1 for injection. Furthermore, when theresin has solidified completely and the screw 7 is stopped, no resinreaction force acts on the screw 7, so that by neglecting the resinreaction force, the servomotor M1 for injection can be moved to theintermediate position of overall backlash in the power transmittingsystem between the screw 7 and the servomotor M1 for injection.

These two states are the same in that the servomotor M1 for injectioncan rotate freely in the range of backlash without affecting theoperation of the screw 7. In either case, the servomotor M1 forinjection is positioned at the intermediate position of overall backlashin the power transmitting system between the screw 7 and the servomotorM1 for injection. That is, the servomotor M1 for injection can rotate inboth the normal and reverse directions by a half amount of overallbacklash amount, for example, Da/2, without affecting the position ofthe screw 7 at all, in other words, without causing the screw 7 to beaffected by a resin reaction force.

However, as described above, in order to clarify the effect of the resinreaction force on the screw 7, it should be noted that, actually, theservomotor Ml for injection and the screw 7 are not connected directlyto each other, and there are various components such as the timing beltfor driving the ball screw 8, ball screw 8, and ball nut 9 on the powertransmitting path between the servomotor M1 for injection and the screw7, and that the cumulative value of the backlashes occurring among thesecomponents acts as the overall backlash amount. Therefore, for example,even if the screw 7 is moved from the second stop position to theintermediate position Pm and then the servomotor M1 for injection isrotated in the reverse direction by a half amount of overall backlashamount, the axial contact between the ball screw 8 and the ball nut 9 isnot always cut off, and the contact between the ball screw 8 and theball nut 9 is sometimes maintained.

This is because when the servomotor M1 for injection is rotated in thereverse direction, the backlashes are eliminated in the order of thebacklash between the timing belt for driving the ball screw 8 and theservomotor M1 for injection, the backlash between the timing belt andthe ball screw 8, and the backlash between the ball screw 8 and the ballnut 9, and the backlash between the ball screw 8 and the ball nut 9 iseliminated last, so that sometimes the ball screw 8 is not rotatedadequately in the reverse direction.

In particular, according to the present embodiment, the final operationfor positioning to the intermediate position Pm is performed by movingthe screw in the negative direction, and so, if the axial contactbetween the ball screw 8 and the ball nut 9 is maintained in a statesuch that the servomotor M1 for injection is rotated in the reversedirection by a half amount of backlash amount, there is a possibilitythat a stress acts on the load cell 10. Naturally, in order to removeall external forces from the load cell 10, the axial contact between theball screw 8 and the ball nut 9 must be cut off.

Thus, when the flag F is "0" and the resin is in a melted state (StepS36), the CPU 115 for CNC, which has completed the movement of the screw7 to the intermediate position Pm on which no resin pressure is exerted,releases the torque limit of the servomotor M1 for injection (Step S37),and determines the initial value A of amplitude by adding a preset valueα to the magnitude of movement (Qb-Pm) by which the motor M1 can be madeto rotate in the normal and reverse direction on the basis of theposition of the servomotor M1 for injection corresponding to theintermediate position of overall backlash amount (Step S38).

Since (Qb-Pm)=(Pm-Qf), A=Qb-Pm+α=Pm-Qf+α.

When the resin has melted and the screw 7 can be moved, the value of(Qb-Qf) is larger than the overall backlash amount Da, so that, in sucha case, the screw 7 itself actually reciprocates. It seems that such anoperation, causing the screw 7 itself to reciprocate, conflicts with thetechnical concept such that an external force exerted on the screwpusher plate 3 from the screw 7 is removed to stably stop the screwpusher plate 3 by positioning the screw 7 at the position Pm at whichthe resin reaction force balances, and the axial contact between theball screw 8 and the ball nut 9 is cut off using the overall backlashbetween the servomotor M1 for injection and the ball nut 9, therebyremoving all external forces acting on the load cell 10; however, thisoperation actually is effective for surely removing the resin reactionforce acting on the screw 7 (since, at this stage, the torque limit ofthe servomotor M1 for injection is canceled as described previously, thescrew 7 can be moved beyond the advance limit Qf or the retreat limit Qbat the time of torque limit T2).

Theoretically, the resin reaction force acting on the screw 7 can beremoved completely by moving the screw 7 to the intermediate position Pmbetween the first stop position Qf and the second stop position Qb.However, in each of the screw movement processes such as the screwmovement from the initial current position Ps of the screw to the firststop position Qf (movement of a4 shown in FIG. 3), the screw movementfrom the first stop position Qf to the initial current position Ps ofthe screw (movement of a5 shown in FIG. 3), the screw movement from theinitial current position Ps of the screw to the second stop position Qb(movement of a6 shown in FIG. 3), and the screw movement from the secondstop position Qb to the intermediate position Pm, especially in a statein which the resin does not solidify completely, the movement of thescrew 7 itself brings about a change in the relationship between theinjection cylinder 6, the screw 7, and the resin, so that, finally, evenif the screw 7 is moved to the intermediate position Pm, at which theresin pressure is supposed to balance, the resin reaction force cannotactually be prevented completely from acting on the screw 7 in somecases.

In effect, in order to determine the screw position at which the resinpressure balances, the limit position Qf to which the screw can advanceand the limit position Qb to which the screw can retreat must bedetected by moving the screw 7. However, the movement of the screw 7itself gives rise to a problem such as the change in the environmentalcondition of the resin.

Thus, the screw 7 is positively reciprocated with a small amplitude,with respect to the intermediate position Pm being the center of thereciprocation, in order to make the melted resin snugly fit theinjection cylinder 6 and the screw 7, thereby removing the residualresin reaction force acting on the screw 7 more effectively. Since thecenter of reciprocation is always the intermediate position Pm, thestate of melted resin is changed so that the resin pressure acting onthe screw 7 becomes zero in a state in which the screw 7 is positionedat Pm. As a means for determining the value A to set a value slightlyhigher than (Qb-Qf), (Qb-Qf) being a half of the distance between thefirst stop position and the second stop position, as the initial valueof amplitude, a method in which the calculation of (Qb-Pm)+α a is madeand a method in which the calculation of (Qb-Pm)·β is made are available(however, β is a value slightly larger than 1).

If it is found in Step S36 that the flag F is "1" and that the resin hassolidified or has a high viscosity, a torque limit value T3 is set tolimit the output of the servomotor M1 for injection to a low output, andthe amplitude A is set to (Qb-Pm) (Steps S39 and S40). The torque limitvalue T3 should have the relationship of T3>T2.

The CPU 115 for CNC, after having determined the initial value ofamplitude A, outputs a movement command to advance the screw 7 from theintermediate position Pm by the amplitude A and verifies the movement ofthe screw 7 to the commanded position (Steps S41 and S42, movement of a8shown in FIG. 3), and then outputs a movement command to retreat thescrew 7 from the intermediate position Pm by the amplitude A andverifies the movement of the screw 7 to the commanded position (StepsS43 and S44, movement of a9 shown in FIG. 3).

One cycle of the reciprocating motion, with respect to the intermediateposition being the center, is performed by the processings in Steps S41through S44, and the magnitude of amplitude is A or the initial value of(Qb-Pm)+α.

Next, the CPU 115 for CNC determines whether or not the current value ofamplitude A is lower than a preset value δ (Step S45). If it is foundthat the current value is not smaller than δ, this amplitude A ismultiplied by a preset value B to obtain a new amplitude A (Step S46),and the processing in Step S41 and the subsequent steps are repeatedlyexecuted in the same way as described above based on this amplitude A,thereby causing one cycle of reciprocating operation of the screw 7 tobe performed with respect to the intermediate position Pm being thecenter (movements of a10 and all shown in FIG. 3).

Since the preset value B is a preset value for gradually decreasing theamplitude of the screw 7, the allowable setting range thereof isnaturally 0<B<1, and in this embodiment, a value of 0.9 is employed.Also, since this reciprocating motion must be continued until theamplitude becomes smaller than 1/2 of the backlash amount Da, the presetvalue δ must be a value not larger than Da/2.

During the repeated execution of the processing in Steps S41 to S46, thevalue of the amplitude A decreases gradually. When this value becomeslower than 1/2 of the backlash amount Da, the reciprocating motion ofthe screw 7 itself stops at the intermediate position being the centerof the reciprocating motion. Then, a state is established in which themelted resin snugly fits the injection cylinder 6 and the screw 7 sothat the resin reaction force will not act at all on the screw 7,thereby enabling the screw pusher plate 3 to be stopped completely andstably at this position.

If the value of the amplitude A becomes lower than a half of thebacklash amount, Da/2, the ball screw 8 cannot be rotated to a backlashelimination position at which the normal and reverse rotation of theball screw 8 is directly transmitted to the ball nut 9 as the advanceforce of the screw 7 or a backlash elimination position at which thenormal and reverse rotation of the ball screw 8 is directly transmittedto the ball nut 9 as the retreat force of the screw 7, so that thesubstantial axial contact between the ball screw 8 and the ball nut 9 iscut off.

In this state, the ball screw 8 is completely separated from the ballnut 9 in the axial direction, that is, the ball screw 8 is not incontact with the ball nut 9, so that neither a force with which the ballnut 9 pushes the load cell 10 nor a force with which the ball nut 9pulls the load cell 10 will act on the ball screw 8. Further, the screw7 itself is not subjected to the resin reaction force at all, and thescrew pusher plate 3, which fixes the load cell 10, rests completely andstably, so that there is no substantial disturbance acting on the loadcell 10.

In this embodiment, in which the load cell 10 is arranged between theball nut 9 and the screw pusher plate 3 or the final position on thepower transmitting path, when the magnitude of the amplitude becomessmaller than the overall backlash amount Da, the reciprocating motioncan be stopped immediately to make zero adjustment of the load cell 10.Actually, however, the preset value δ is set to a value lower than Da/2taking into account some allowance for safety.

The CPU 115 for CNC, after detecting through the processing in Step S45that the current value of the amplitude A is lower than the preset valueδ and the reciprocating motion is finished, reads the detected currentvalue of the pressure of load cell 10 through the CPU 107 for pressuremonitoring, inverts the sign of the value, and stores the value in thecorrection value storage region of the RAM 102 (Step S47), therebycompleting the zero adjustment processing.

For example, if the detected pressure of the load cell 10 is 10 kg/cm²despite the fact that the current value of amplitude A is lower than thepreset value δ and the disturbance acting on the load cell 10 is zero,this means that a stress of 10 kg/cm² acts on the load cell 10 caused byerror factors of the load cell 10 itself including the plasticdeformation of coating material and the change in air bubble state, sothat -10 kg/cm² is stored in the correction value storage region of theRAM 102, and adjustment is made so that the pressure detected by the CPU107 for pressure monitoring becomes zero when the disturbance acting onthe load cell 10 is zero, that is, when a value of 10 kg/cm² isoutputted from the load cell 10. As already described, the valuedetected as the resin reaction force by the CPU 107 for pressuremonitoring through the sampling of the injection holding pressure andback pressure is a value obtained by adding a correction value to theoutput of the A/D converter 106. Therefore, the resin reaction force canbe detected accurately regardless of the causes for error factors of theload cell 10 itself including the plastic deformation of coatingmaterial and the change in air bubble state.

However, the resin reaction force described here is a disturbance actingon the load cell 10 itself, and not the resin reaction force itselfacting on the screw 7. When the resin reaction force itself acting onthe screw 7 is to be detected accurately, the frictional resistances A1and A2 acting between the tie rod 2 and the screw pusher plate 3, africtional resistance acting between the screw 7 and the injectioncylinder 6, etc. are hindrances to accurate detection, so that thecorrection value must be offset in a direction in which the error causedby these frictional resistances can be eliminated depending on advancingor retreating movement of the screw 7, that is, the direction of thestress or tension acting on the load cell 10.

If the detected pressure of the load cell 10, which is detected in theprocessing of Step S47, deviates extremely from zero being thereference, there is the possibility that some serious abnormality suchas buckling of the load cell body 11 has occurred. Thus, if zeroadjustment is made in such a state, the load cell 10 will not be able tofunction as expected, so that an abnormality detection signal should beoutputted instead of the zero adjustment.

If the absolute value of the detected pressure of the load cell 10,which is read in the processing in Step S47, is not larger than a presetvalue and the deviation from zero point is of a negligible degree, theupdating and storing of correction value, that is, a substantialadjusting operation may be omitted.

Further, in this embodiment, a file for storing the history ofadjustment is provided in the nonvolatile memory 114 (see FIG. 8) sothat the correction amount, correction date, number of shots duringoperation, integrated operation time of injection molding machine, etc.are stored in this file when the processing in Step S47 is finished. Thefile content can be displayed on the screen of the manual data inputdevice 119 with display by the function menu selecting operation, sothat the file content can be referred to whenever necessary.

Described above as an embodiment is the case where the load cell 10 isarranged between the ball nut 9 and the screw pusher plate 3, which isthe final position on the power transmitting path. In some cases,however, the load cell 10 can be installed at a position closer to theservomotor M1 for injection on the power transmitting path, depending onthe configuration of injection mechanism.

Comparing the case where the load cell 10 is located at the finalposition on the power transmitting path with the case where the loadcell 10 is located at a position closer to the servomotor M1 forinjection, the distribution of the overall backlash between the sectionsdivided by the load cell 10 differs significantly depending on thelocation of the load cell 10 on the power transmitting path. Forinstance, there is little backlash between the load cell 10 and thescrew 7 in the above-mentioned case, whereas a large backlash is presentbetween the load cell 10 and the screw 7 in the case where the load cell10 is installed at a position closer to the servomotor M1 for injection.

In the case where the load cell 10 is located at the position closer tothe servomotor M1 for injection, even if the value of amplitude A islower than 1/2 of the overall backlash amount Da, the external forceacting on the load cell 10 does not necessarily become zero. However, asdescribed previously, by continuing the reciprocating motion withoutstopping the reciprocating motion even after the magnitude of amplitudebecomes smaller than 1/2 of the overall backlash amount, all thebacklashes with respect to various components constituting the powertransmitting path can be moved to the neutral positions of thebacklashes, and the dynamic connection among the components can be cutoff. Therefore, as long as δ is set at a low value, zero adjustment canbe made by removing the external forces acting on the load cell 10regardless of the location of the load cell 10 on the power transmittingpath. Needless to say, from the viewpoint of the detection accuracy, itis most desirable to install the load cell 10 at a closest possibleposition to the screw 7 on the power transmitting path.

Further, if the aforesaid zero adjustment processing is executed whenthe automatic purge item is selected by the function menu using themanual data input device 119 with display, the zero adjustment of theload cell 10 can be made without fail at proper time intervals such asthose required for the change of molded material. It is preferable tomake zero adjustment after the resin in the injection cylinder 6 isdischarged completely by executing the purging processing. However, inthe case where the configuration is designed so that the automatic purgecan be interrupted by actuation of an emergency switch for stopping allthe functions of the injection molding machine, the execution of zeroadjustment processing sometimes becomes impossible when the automaticpurge is stopped for the convenience of the operator. Therefore, it ispreferable to establish a sequence such that the zero adjustmentprocessing can be executed by interrupting the purge operation beforethe start of automatic purge or after finishing the automatic purge ofthe first time, second time, etc. which have to always be executed.

In addition, the same processing operation can be applied to a pressuredetector of an ejector rod or a movable platen.

According to the present invention, the external forces acting directlyon movable members are removed completely by reciprocating the drivesource of the movable members; backlashes having equal plays in themoving directions of the movable member are provided between thecomponent on the power transmitting path, which directly drives themovable member on the power transmitting path ranging from the drivesource to the movable member, and the movable member, by which thesubstantial force transmission between the movable member and thecomponent is cut off; and the external pressure acting on the pressuredetector is removed completely so that the zero adjustment of thepressure detector can be made in a state in which the disturbance iszero. Therefore, the zero adjustment of the pressure detector can bemade simply and surely without requiring cumbersome work such asdisassembling the injection molding machine for disengaging the pressuredetector from other members of the injection.

Also, the processing operation necessary for the correction of thepressure detector is stored in the control unit of the injection moldingmachine so that the correction processing of the pressure detector canbe executed together with the automatic purging operation. Therefore,the zero adjustment of the pressure detector can be made without fail atproper time intervals such as at the changing of molded material, andthe injection holding pressure and the back pressure during metering canalways be sampled with a properly calibrated pressure detector.

Further, an abnormality detection signal is generated when the absolutevalue of the correction amount of the output of pressure detectorexceeds the preset amount. Therefore, even a serious abnormality, whichcannot be corrected by a mere zero adjustment, can be detected.

Also, the history of zero adjustment including the date and correctionamount of adjustment is stored in the control unit of the injectionmolding machine, and the stored history can be displayed. Therefore, thedeterioration in performance and serious abnormality of the pressuredetector can be found more easily.

We claim:
 1. A method of adjusting a zero point of a pressure detectorof an injection molding machine, for detecting a pressure exerted on amovable member of the machine, comprising the steps of:(a) starting areciprocating motion of the movable member, said motion having agradually decreasing amplitude relative to a center point; (b) stoppingsaid reciprocating motion when said amplitude becomes a value not higherthan a predetermined value; (c) reading an output value of said pressuredetector when said reciprocating motion is stopped in said step (b); and(d) correcting the output of said pressure detector based on the valueread in said step (c).
 2. A zero point adjusting method for a pressuredetector of an injection molding machine according to claim 1, furthercomprising the steps of:(e) outputting a command to a drive source fordriving said movable member, for moving said movable member by apredetermined amount from a current position of said movable member bysetting an output of said drive source to a low level, and thendetecting a stop position of said movable member to store said stopposition as a first stop position; and (f) outputting a command formoving said movable member to said drive source in a reversed directionby said preset amount from a current position of said movable member,and then detecting a stop position of said movable member to store saidstop position as a second stop position, wherein said reciprocatingmotion in said step (a) is started with an intermediate point betweensaid first and second stop positions as a center of said motion.
 3. Azero point adjusting method for a pressure detector on an injectionmolding machine according to claim 2, wherein said reciprocating motionin said step (a) is started with an initial value of said amplitude notlarger than 1/2 of a distance between said first and second stoppositions and not smaller than 1/2 of an amount of backlash producedbetween said movable member and said drive source.
 4. A zero pointadjusting method for a pressure detector on an injection molding machineaccording to claim 2, wherein said reciprocating motion in said step (a)is started with an initial value of said amplitude, which is obtained byadding a preset value to 1/2 of the distance between said first andsecond stop positions.
 5. A zero point adjusting method for a pressuredetector of an injection molding machine according to claim 2, whereinsaid reciprocating motion in said step (a) is started with an initialvalue of said amplitude, which is obtained by multiplying 1/2 of thedistance between said first and second stop positions by a number largerthan
 1. 6. A zero point adjusting method for a pressure detector on aninjection molding machine according to claim 1, wherein said step (b)includes a step of stopping said reciprocating motion when saidamplitude becomes smaller than 1/2 of the amount of backlash producedbetween said movable member and said drive source.
 7. A zero pointadjusting method for a pressure detector on an injection molding machineaccording to claim 1, wherein said drive source for driving said movablemember comprises a servomotor having output limiting means.
 8. A zeropoint adjusting method for a pressure detector on an injection moldingmachine according to claim 1, wherein said movable member comprises aninjection screw.
 9. A zero point adjusting method for a pressuredetector of an injection molding machine according to claim 8, includingthe steps of carrying out an automatic purging and automaticallyexecuting said steps (a) to (d).
 10. A zero point adjusting method for apressure detector on an injection molding machine according to claim 1,including a step of outputting an abnormality detection signal when anabsolute value of a correction amount of the output value of saidpressure detector exceeds a preset amount.
 11. A zero point adjustingmethod for a pressure detector of an injection molding machine accordingto claim 1, further comprising a step of storing a history of the zeroadjustment including a date and a correction amount of the output valueof said pressure detector in a controller of the injection moldingmachine and displaying the stored history.
 12. An apparatus foradjusting a zero point of a pressure detector of an injection moldingmachine, for detecting a pressure exerted on a movable member of themachine, comprising:reciprocating motion control means for starting areciprocating motion of the movable member, said motion having agradually decreasing amplitude relative to a center point, and forstopping said reciprocating motion when said amplitude becomes a valuenot higher than a predetermined value; and correcting means for readingan output value of said pressure detector when said reciprocating motionis stopped, and correcting the output of said pressure detector based onthe read value.
 13. A zero-point adjusting apparatus for a pressuredetector of an injection molding machine according to claim 12, whereinsaid reciprocating motion control means outputs a command to a drivesource for driving said movable member, for moving said movable memberby a predetermined amount from a current position of said movable memberby setting an output of said drive source to a low level, and thendetects a stop position of said movable member to store said stopposition as a first stop position, and outputs a command for moving saidmovable member to said drive source in a reversed direction by saidpreset amount from a current position of said movable member, and thendetects a stop position of said movable member to store said stopposition as a second stop position, andsaid reciprocating motion isstarted with an intermediate point between said first and second stoppositions as a center of said motion.
 14. A zero-point adjustingapparatus for a pressure detector of an injection molding machineaccording to claim 13, wherein said reciprocating motion control meansstarts said reciprocating motion with an initial value of said amplitudenot larger than 1/2 of a distance between said first and second stoppositions and not smaller than 1/2 of an amount of backlash producedbetween said movable member and said drive source.
 15. A zero-pointadjusting apparatus for a pressure detector of an injection moldingmachine according to claim 13, wherein said reciprocating motion controlmeans starts said reciprocating motion with an initial value of saidamplitude, which is obtained by adding a preset value to 1/2 of thedistance between said first and second stop positions.
 16. A zero-pointadjusting apparatus for a pressure detector of an injection moldingmachine according to claim 13, wherein said reciprocating motion controlmeans starts said reciprocating motion with an initial value of saidamplitude, which is obtained by multiplying 1/2 of the distance betweensaid first and second stop positions by a number larger than
 1. 17. Azero-point adjusting apparatus for a pressure detector of an injectionmolding machine according to claim 12, wherein said reciprocating motioncontrol means stops said reciprocating motion when said amplitudebecomes smaller than 1/2 of the amount of backlash produced between saidmovable member and said drive source.
 18. A zero-point adjustingapparatus for a pressure detector of an injection molding machineaccording to claim 12, wherein said movable member is driven by aservomotor having output limiting means.
 19. A zero-point adjustingapparatus for a pressure detector of an injection molding machineaccording to claim 12, wherein said movable member comprises aninjection screw.
 20. A zero-point adjusting apparatus for a pressuredetector of an injection molding machine according to claim 19, whereinsaid reciprocating motion control means carries out an automatic purgingand executes the start and stop of said reciprocating motionautomatically.
 21. A zero-point adjusting apparatus for a pressuredetector of an injection molding machine according to claim 12, furthercomprising abnormality detection signal output means for outputting anabnormality detection signal when an absolute value of a correctionamount of the output value of said pressure detector exceeds a presetamount.
 22. A zero-point adjusting apparatus for a pressure detector ofan injection molding machine according to claim 12, further comprising adisplay device for storing a history of the zero adjustment including adate and a correction amount of the output value of said pressuredetector in a controller of the injection molding machine and displayingthe stored history.